1. Field of the Invention
The present invention relates to apparatus and methods for use in patterning the metallization of a semiconductor device. More particularly, this invention pertains to the utilization of a "hard" mask for patterning of metallizations that include aluminum alloys.
2. Description of the Prior Art
Semiconductor devices generally require a network of metallizations at their upper surfaces that serve as gates and interconnecting conductors for device control, activation and output. For example, in a so-called charge coupled device (CCD) conductive metallizations provide means for "reading out" charge accumulated in localized regions of the device as a consequence and measure of incident radiation. The arrangement or "patterning" of metallizations must be extremely accurate. This is especially true in the case of VLSI (very large scale integration) devices that may possess densities in excess of 10.sup.5 transistors per chip.
In addition to accuracy of location, the optimum operation of such devices requires metallizations of consistent line widths and the absence of "undercutting". The former is exceedingly difficult to attain in the fabrication of VLSI devices that commonly require gate widths of less than two (2) microns. An extremely small deviation from nominal width may result in significant and undesired localized effects. The latter characteristic can introduce deleterious stray capacitances that produce device and system failures.
The "standard" metal for use in present day devices is aluminum. This metal and its selected alloys possess the lowest resistivity of all materials that have been found to be practical. That is, the selected aluminum-based alloys have proven to be amenable to commercial etching and like processes.
In the past, patterning has generally been accomplished by etching processes in which a layer of metallization is deposited upon a semiconductor substrate. A layer of photoresist is then deposited atop the metallization and photolithographically developed and removed by means of an acetate bath or the like. Thereafter a (liquid or gaseous) etchant is introduced that attacks the exposed portions of metallization, leaving the desired pattern atop the substrate.
A relatively long time is required to etch the metallization, since a relatively thick layer of photoresist must be employed to protect the pattern throughout etching since present-day etchants are highly corrosive and will tend to dissolve the photoresist that remains to protect the underlying metallization causing corrosion of the underlying metallization. For example, a 1.5 micron (15,000 Angstrom) thick layer of photoresist is normally required to protect a 7,000 Angstroms thick layer of metallization during etching. The photoresist traps highly corrosive etchant (solution or gas) ions. Therefore, while a thick layer of photoresist protects the metallization, the very thickness of the layer materially increases the risk of corrosion after the pattern has been formed. Further, the thicker the layer of photoresist, the greater the difficulty of maintaining a very narrow physical aperture for etching a correspondingly-narrow line of metallization.